feat(explore): point at source this session already sent, don't send it twice (CG-18)

A later explore call re-served whatever it re-ranked, so on the #1500 report
the 4th call spent its envelope on the spine the 1st call had already
delivered. CG-17 recorded what was served; this acts on it.

What a withheld span becomes is the whole design: a POINTER, never a silence.
An insufficient-feeling response is what sends an agent to Read, and one or
two of those early in a session teach it to abandon codegraph — so the
replacement names the file, the symbols and the line spans, and says both
that the source came from THIS conversation and that the file has not changed
since.

- Content fingerprint, not the drift flag, gates it. They answer different
  questions: two calls inside one drift window served the same current bytes,
  while a file edited AND re-synced between calls is never "stale" and yet
  the agent's copy is now wrong. An edited file re-emits in full.
- Only a covered run of >= 8 lines is replaced, and a remainder under 160
  chars folds into the pointer. Below those the pointer costs more than the
  source and the block reads as shredded — a fence holding `228\t` is a
  broken-looking response, which is the expensive failure.
- The reclaimed bytes go to files the agent has NOT seen, two ways: a smaller
  `sourceSpent` hands slack down CG-21's carry-forward pool, and a fully
  back-referenced file gives up its maxFiles slot the way a cliffed one does.
  Within a file, the cluster shrink now reads the DEDUPED length, so it never
  drops new symbols to make room for source it isn't sending.
- If dedup suppresses everything and nothing new takes its place, the top
  suppressed file is spliced back in whole. An all-pointer response is the
  shape that reads as "codegraph found nothing"; one re-served file is the
  cheaper mistake.

Kill switch: CODEGRAPH_EXPLORE_DEDUP=0.

src/mcp/explore-session-state.ts

@@ -70,6 +70,13 @@ export interface ExploreFileEmission {
   ranges: ExploreLineRange[];
   /** Source chars emitted for this file (excludes headers / fences). */
   bytes: number;
+  /**
+   * Identity of the bytes those ranges were sliced from (CG-18). Cross-call
+   * dedup withholds a span only when the file still hashes to this, so an edit
+   * between two calls re-serves instead of pointing at source the agent holds a
+   * now-wrong copy of. Absent = unprovable, which dedup treats as "re-serve".
+   */
+  fingerprint?: string;
   /** Set when ranges were dropped to stay under the per-file bound. */
   rangesTruncated?: boolean;
 }
@@ -309,6 +316,7 @@ export class ExploreSessionState {
       .map((f) => {
         const { ranges, truncated } = coalesceRanges(f.ranges ?? []);
         const out: ExploreFileEmission = { path: f.path, ranges, bytes: Math.max(0, f.bytes || 0) };
+        if (typeof f.fingerprint === 'string' && f.fingerprint) out.fingerprint = f.fingerprint;
         if (truncated) out.rangesTruncated = true;
         return out;
       });